Structure and viscoelasticity of mixed micelles formed by poly(ethylene oxide) end capped with alkyl groups of different length

Hybridization of Poly(oxazoline) and Poly(ethylene oxide)-Based Amphiphilic Copolymers into Thermosensitive Mixed Micelles of Tunable Cloud Point

This paper reports the development in aqueous solution of mixed micelles of tunable cloud point temperature through blending in various proportions of two copolymers of different chemical natures. For that purpose, a lipid-b-poly(2-isopropyl-2-oxazoline) (lipid-b-P(iPrOx)) copolymer, self-assembling into thermosensitive micelles that phase-separate above a cloud point temperature of 38 °C, was blended in various proportions with commercial C₁₈-b-PEO_x. The latter was constituted of a hydrophobic saturated C₁₈ chain and a hydrophilic poly(ethylene oxide) (PEO) block with varying polymerization degrees (x) and does not have any thermosensitive properties on the studied temperature range for any value of x. The different blends were thoroughly characterized by light scattering and UV-visible spectroscopy, revealing that hybridization between both copolymers always occurred, independent of the PEO block length. The resulting mixed micelles present T_CP values progressively increasing with the C₁₈-b-PEO_x proportion, from 38 to 61 °C. This study demonstrates the relevance of the blending approach to tune the phase separation of micellar systems by formulation rather than by more tedious synthetic efforts. Shifting T_CP through this approach extends the range of temperature where lipid-b-P(iPrOx) can find an application.

Conformance Control in Oil Reservoirs by Citric Acid-Coated Magnetite Nanoparticles

Reservoir conformance control methods may significantly improve enhanced oil recovery technologies through reduced water production and profile correction. Excessive water production in oil and gas reservoirs leads to severe problems. Water shutoff and conformance control are, therefore, financially and environmentally advantageous for the petroleum industry. In this paper, water shutoff performance of citric acid-coated magnetite (CACM) and hematite nanoparticles (NPs) as well as polyacrylamide polymer solution in a heterogeneous and homogeneous two-dimensional micromodel is compared. A facile one-step technique is used to synthesize the CACM NPs. The NPs, which are reusable, easily prepared, and environmentally friendly, are characterized using Fourier-transform infrared spectroscopy, field emission scanning electron microscopy, dynamic light scattering, and X-ray diffraction. The results confirm uniform spherical Fe₃O₄ NPs of an average diameter of 40 nm, well coated with citric acid. CACM NPs provide a high pressure drop coupled with an acceptable resistance factor and residual resistance factor owing to NP arrangement into a solid-/gel-like structure in the presence of a magnetic field. A resistance factor and a residual resistance factor of 3.5 and 2.14, respectively, were achieved for heavy oil and the heterogeneous micromodel. This structure contributed to an appreciable plugging efficiency. CACM NPs respond to ∼1000 G of magnetic field intensity and display a constant resistance factor at intensities between 4500 and 6000 G. CACM NPs act as a gel, forming a solid-/gel-like structure, which moves toward the magnetic field and thereby shuts off the produced water and increases the oil fraction. The findings of this study suggest the ability to shut off water production using specially designed magnetic field-responsive smart fluids. The application would require innovative design of field equipment.

Spherical Micelles with Nonspherical Cores: Effect of Chain Packing on the Micellar Shape

Self-assembly of amphiphilic polymers into micelles is an archetypical example of a "self-confined" system due to the formation of micellar cores with dimensions of a few nanometers. In this work, we investigate the chain packing and resulting shape of C n -PEOx micelles with semicrystalline cores using small/wide-angle X-ray scattering (SAXS/WAXS), contrast-variation small-angle neutron scattering (SANS), and nuclear magnetic resonance spectroscopy (NMR). Interestingly, the n-alkyl chains adopt a rotator-like conformation and pack into prolate ellipses (axial ratio ϵ ≈ 0.5) in the "crystalline" region and abruptly arrange into a more spheroidal shape (ϵ ≈ 0.7) above the melting point. We attribute the distorted spherical shape above the melting point to thermal fluctuations and intrinsic rigidity of the n-alkyl blocks. We also find evidence for a thin dehydrated PEO layer (≤1 nm) close to the micellar core. The results provide substantial insight into the interplay between crystallinity and molecular packing in confinement and the resulting overall micellar shape.

Structure and thermodynamics of mixed polymeric micelles with crystalline cores: tuning properties via co-assembly

We investigate micelles formed by mixtures of n-alkyl-poly(ethylene oxide) block copolymers, Cn-PEO, with different alkyl block lengths in aqueous solution. This model system has previously been used to shed light on the interplay between exchange kinetics and crystallinity in self-assembling systems [König et al., Phys. Rev. Lett., 2019, 122, 078001]. Now we report on the structure and thermodynamics of these micelles by combining results from small-angle X-ray scattering, differential scanning calorimetry and volumetric measurements. We show that mixed micelles are formed despite the fact that length-mismatched n-alkanes of similar weights in bulk tend to demix below the crystallization temperature. Instead, the system exhibits similar properties as single-component micelles but with a modulated melting region. Interestingly, the melting point depression due to self-confinement within the micellar core can be approximately described by a generalized Gibbs-Thomson equation, similar to single-component micelles [Zinn et al. Phys. Rev. Lett., 2014, 113, 238305]. Furthermore, we find a novel scaling law for these micelles where, at least for larger n, the aggregation number scales with the third power of the length of the hydrophobic block, Nagg ∝ n3. Possibly, there might be a cross-over from the conventional Nagg ∝ n2 behaviour around n ≈ 19. However, the reason for such a transition as well as the strong n dependence remains a challenge and requires more theoretical work.

Simulations Study of Single-Component and Mixed n-Alkyl-PEG Micelles

Here, we study one-component and mixed n-alkyl-poly(ethylene glycol) (C mE n) micelles with varying poly(ethylene glycol) (PEG) chain lengths n using coarse-grained molecular simulations. These nonionic alkyl-PEG surfactants and their aggregates are widely used in bio and chemical technology. As expected, the simulations show that increasing the PEG chain length decreases the alkyl-PEG micelle core diameter and the aggregation number but also enhances PEG chain penetration to the core region and spreads the micelle corona. Both the core and corona density are heavily dependent on the PEG chain length and decrease with increasing PEG length. Furthermore, we find that the alkyl-PEG surfactants exhibit two distinct micellization modes: surfactants with short PEG chains as their hydrophilic heads aggregate with the PEG heads relatively extended. Their aggregation number and the PEG corona density are dictated by the core carbon density. For longer PEG chains, the PEG sterics, that is, the volume occupied by the PEG head group, becomes the critical factor limiting the aggregation. Finally, simulations of binary mixtures of alkyl-PEGs of two different PEG chain lengths show that even in the absence of core-freezing, the surfactants prefer the aggregate size of their single-component solutions with the segregation propelled via enthalpic contributions. The findings, especially as they provide a handle on the density and the density profile of the aggregates, raise attention to effective packing shape as a design factor of micellar systems, for example, drug transport, solubilization, or partitioning.

Blending block copolymer micelles in solution; Obstacles of blending

Amphiphilic block copolymers can assemble into a variety of structures on the nanoscale in selective solvent. The micelle blending protocol offers a simple unique route to reproducibly produce polymer nanostructures. Here we expand this blending protocol to a range of polymer micelle systems and self-assembly routes. We found by exploring a range of variables that the systems must be able to reach global equilibrium at some point for the blending protocol to be successful. Our results demonstrate the kinetics requirements, specifically core block glass transition temperature, Tg, and length of the block limiting the exchange rates, for the blending protocol which can then be applied to a wide range of polymer systems to access this simple protocol for polymer self-assembly.

Sol–gel reversible metallo-supramolecular hydrogels based on a thermoresponsive double hydrophilic block copolymer

A new thermoresponsive double hydrophilic block copolymer bearing a terpyridine moiety formed a hydrogel with a sol–gel thermoreversible transition in the presence of Fe²⁺ ions.

Multiscale Molecular Dynamics Simulations of Model Hydrophobically Modified Ethylene Oxide Urethane Micelles

The flower-like micelles of various aggregation numbers of a model hydrophobically modified ethylene oxide urethane (HEUR) molecule, C16E45C16, and their corresponding starlike micelles, containing the surfactants C16E22 and C16E23, were studied by atomistic and coarse-grained molecular dynamic (MD) simulations. We used free energies from umbrella sampling to calculate the size distribution of micelle sizes and the average time for escape of a hydrophobic group from the micelle. Using the coarse-grained MARTINI force field, the most probable size of the model HEUR molecule was thereby determined to be about 80 hydrophobes per micelle and the average hydrophobe escape time to be about 0.1 s, both of which are consistent with previous experimental studies. Atomistic simulations reveal that hydrogen bond formation and the mean lifetime of hydration waters of the poly(ethylene oxide) (or PEO) groups are location-dependent in the HEUR micelle, with PEO groups immediately adjacent to the C16 groups forming the fewest hydrogen bonds with water and having hydration waters with longer lifetimes than those of the PEO groups located further away from the C16 groups.

Surfactant or block copolymer micelles? Structural properties of a series of well-defined n-alkyl-PEO micelles in water studied by SANS

Here we present an extensive small-angle neutron scattering (SANS) structural characterization of micelles formed by poly(ethylene oxide)-mono-n-alkyl ethers (Cn-PEOx) in dilute aqueous solution. Chemically, Cn-PEOx can be considered as a hybrid between a low-molecular weight surfactant and an amphiphilic block copolymer. The present system, prepared through anionic polymerization techniques, is better defined than other commercially available polymers and allows a very precise and systematic testing of the theoretical predictions from thermodynamical models. The equilibrium micellar properties were elaborated by systematically varying the n-alkyl chain length (n) at constant PEO molecular weight or increasing the soluble block size (x), respectively. The structure was reminiscent of typical spherical star-like micelles i.e. a constant core density profile, ∼r(0), and a diffuse corona density profile, ∼r(-4/3). Through a careful quantitative analysis of the scattering data, it is found that the aggregation number, Nagg initially rapidly decreases with increasing PEO length until it becomes independent at higher PEO molecular weight as expected for star-like micelles. On the other hand, the dependency on the n-alkyl length is significantly stronger than that expected from the theories for star-like block copolymer micelles, Nagg ∼ n(2) similar to what is expected for surfactant micelles. Hence the observed aggregation behavior suggests that the Cn-PEOx micelles exhibit a behavior that can be considered as a hybrid between low-molecular weight surfactant micelles and diblock copolymer micelles.

Structural characterization of amphiphilic homopolymer micelles using light scattering, SANS, and cryo-TEM

We report the aqueous solution self-assembly of a series of poly(N-isopropylacrylamide) (PNIPAM) polymers end-functionalized with a hydrophobic sulfur-carbon-sulfur (SCS) pincer ligand. Although the hydrophobic ligand accounted for <5 wt% of the overall homopolymer mass, the polymers self-assembled into well-defined spherical micelles in aqueous solution, and these micelles are potential precursors to solution-assembled nanoreactors for small molecule catalysis applications. The micelle structural details were investigated using light scattering, cryogenic transmission electron microscopy (cryo-TEM), and small angle neutron scattering (SANS). Radial density profiles extracted from the cryo-TEM micrographs suggested that the PNIPAM chains formed a diffuse corona with a radially decreasing corona density profile and provided valuable a priori information about the micelle structure for SANS data modeling. SANS analysis indicated a similar profile in which the corona surrounded a small hydrophobic core containing the pincer ligand. The similarity between the SANS and cryo-TEM results demonstrated that detailed information about the micelle density profile can be obtained directly from cryo-TEM and highlighted the complementary use of scattering and cryo-TEM in the structural characterization of solution-assemblies, such as the SCS pincer-functionalized homopolymers described here.

Slow dynamics in transient polyelectrolyte hydrogels formed by self-assembly of block copolymers

Transient polyelectrolyte hydrogels were formed by self-assembly of triblock copolyelectrolytes with a central hydrophilic block, poly(acrylic acid) (PAA), and two hydrophobic end blocks, poly(n-butyl acrylate(50%)-stat-acrylic acid(50%)) [P(nBA(50%)-stat-AA(50%))]. The relaxation of the concentration fluctuations was investigated by dynamic light scattering as a function of the concentration, the pH, the temperature, and the ionic strength. A relatively fast mode was observed at all polymer concentrations caused by cooperative diffusion of the polymers. Above the critical percolation concentration a second slow relaxation mode was observed caused by a linear displacement of small heterogeneities in the network with constant velocity. The relative amplitude of the slow mode was determined by the strength of the electrostatic repulsion. The velocity of the displacement in the transient network is shown to be directly correlated to the terminal relaxation time of the shear modulus and has the same Arrhenius temperature dependence. Both the velocity of the displacement and the mechanical relaxation strongly slow down with decreasing degree of ionization below 0.7 and increasing ionic strength above 0.5 M. A ballistic relaxation process has been reported earlier for colloidal gels, and the present study shows that it can also occur in polymeric networks.